Many digital printing systems are adapted to print on a web of media. In order to provide reliable performance, it is necessary to accurately control the tension in the web of media. The tension in the web of media will be closely related to the stress in the media.
One method for characterizing stress in materials that are at least partially transparent and exhibit photoelasticity is by use of photoelastic measurement devices (i.e., polariscopes). Photoelastic materials exhibit birefringence effects that vary as a function of stress. Photoelastic measurement devices typically measure the stress in the material by characterizing the change in polarization of light which is transmitted through the material.
U.S. Pat. No. 4,093,695 to Heirbaut et al., entitled “Process for making polymeric film,” teaches determining the orientation of the extrusion direction of a plastic film by means of a polariscope.
U.S. Pat. No. 4,847,109 to Shibasaki et al., entitled “Method of a making magnetic recording medium,” discloses an apparatus for making magnetic tape. The illustrated configuration includes two tension sensors, one located upstream and one located downstream of a tensioning roller. Based on the output of the sensors, the tension control mechanism adjusts one of the drive speed of the tensioning roller or the position of the roller to thereby reduce the tension difference measured at the two locations. In some embodiments, the tension sensors are of a type capable of measuring the tension in terms of a change in photoelastic effect.
U.S. Pat. No. 5,288,995 to Strachan, entitled “Electrical measurement apparatus using heterodyne phase conversion techniques,” provides a heterodyne system for analyzing the light from a Moiré fringe interferometer or a polariscope.
U.S. Pat. No. 5,376,999 to Hwang, entitled “Device for minimizing intermediate belt stretch and shrinkage in xerographic copier,” teaches measuring the tension changes in a transport belt of a laser printer using the photoelastic effect.
U.S. Pat. No. 5,663,510 to Niemann et al., entitled “Method and device for compensating tension forces across the width of a moving web,” teaches adjusting the orientation of a web guiding roller based on a measurement of the torque applied to the roller by the web.
U.S. Patent Application Publication 2004/0129867 to Mackey, entitled “Force measurement system using polarization-state modulated optical polarimetry,” discloses a polariscope in which the light beam passes through a liquid crystal variable retarder prior to passing through the photoelastic material for enhancing the measurement of a stain in the photoelastic material.
There remains a need for improved control systems for printing systems that are able to compensate for performance variations that result from changes in stress in the media.